Some fluid systems use valves to control fluid flow. These fluid control valves may include a plug that is seated inside a valve housing. The plug can be moved within the valve housing to adjust the flow of fluid through the valve. As the fluid flows through the valve, notches or embedded corners on the plug surface interact with the fluid and affect the characteristics of the flow. For example, if a significant pressure drop is applied across the fluid control valve, a double-stage valve may be used to stage the pressure let down as the fluid passes over the plug. In such double-stage valves, the fluid flows over notches or embedded corners formed in the surface of the plug, which guide the fluid into a sequence of recovery cavities in the valve housing.
Several factors affect the design of fluid control valves. The type of fluid that is controlled by the valve may influence the materials and dimensions of the valve components. For example, some gasoline refining applications require valves to control the flow of a high-temperature fluid including crude oil and erosive particulates, such as dirt and/or certain catalytic agents. As this erosive fluid flows through the valve, the components may be subjected to temperatures in excess of 500° F. and in extreme cases in excess of 1000° F. and pressure differential across the valve greater than 3000 psi, which result in high fluid velocities at the control surfaces of the-valve. In such instances, a valve plug having specially-dimensioned notches or embedded corners may be used to stage the pressure drop across the valve.
The materials of the valve trim components is another factor to be considered in the design of fluid control valves. The erosion of valve components by high-temperature/high-pressure fluids may lead to significant problems. For example, in some gasoline refining applications, high-temperature crude oil with erosive particulates require replacement of valve plugs made from metal about every six months. Even if the metal can withstand the pressure differentials and tensile stress concentrations imposed at the embedded corners of the valve plug and seat, the erosive fluid can systematically wear away the control surfaces, thereby requiring replacement of the valve components. Rapid erosion of valve components results in significant maintenance and replacement costs.
Other materials may provide better resistance to erosion, but manufacturing costs and operational risks have limited the use of these materials in many commercial applications. In general, manufacturing of metallic valve components is much less costly because machining complex notches and helical threads into metal plugs may be less expensive than machining those same geometries into ceramic plugs. Also, ceramic materials are generally more brittle than metal materials when tensile forces are applied. Because notches, embedded corners, and other complex geometries formed in the surface of the valve plugs may cause tensile stress concentrations in the material, metal components were heretofore believed to be less susceptible to catastrophic failure.